1. Field of the Invention
This invention relates to an ultrasonic pipe inspection apparatus of the type which can be, for example, inserted into the bore of a pipe for the purpose of examining the wall of the pipes for defects, cracks or other discontinuities, and more particularly it relates to the wheel probes used in such apparatus for ultrasonically inspecting the pipe wall.
2. The Prior Art
The usual pipe inspection apparatus for examining, for example, gas pipelines, comprises a vehicle (or a train of vehicles) which is propelled along the pipeline by the gas flow in the pipeline. In one arrangement, the vehicle includes a number of wheels, in which are mounted ultrasonic transducers (usually referred to as "wheel probes") which are urged into contact with the wall of the pipe. In use, the ultrasound generated by the transducers is transmitted across the interface between the wheel probe and the wall of the pipe and into the pipe wall. The ultrasound reflected or refracted from within the pipe wall is in turn received by the transducers within the wheels and subsequently analyzed. This technique requires intimate contact at all times between the wheels and the pipe wall so as to reduce or eliminate losses of the ultrasound signal at the interface between the wheel and the pipe wall.
One form of known wheel probe comprises a hollow wheel assembly having an inflatable tire around its outer circumference, with the ultrasonic transducers being located within the wheel assembly, the sound being transmitted through the inflatable tire. In operation, the tire is urged into contact with the surface to be examined, and an acoustic coupling fluid is maintained between the inflatable tire and the surface to ensure that an adequate coupling is achieved.
In some applications it is impossible, or at least impractical, to use a coupling fluid between the inflatable tire and the surface to be examined. For example, when examining many miles of pipe lines conveying natural gas.
Realizing that there are instances when it is not possible to use a coupling fluid between the tire and the surface to be examined, the natural solution would be to force the inflated tire into dry contact with the surface. However, this requires the tire to be inflated to extremely high pressures to produce the forces required to ensure adequate contact between the tire and the surface.
In addition to the problem of inflating such tires to high pressures when examining many miles of gas pipe lines, other problems also result: the problem of wear of the tires, the problem of punctures caused by encountering sharp obstructions at some weldments, and the problem of tire construction (the material of the tire must be chosen so as not to be deleteriously affected by the gas flowing the pipeline).
An object of the present invention is to provide an improved wheel probe designed to remedy the aforesaid disadvantages and limitations of known wheel probes.
In U.S. patent application Ser. No. 933,031, filed Aug. 11, 1978, now abandoned, there is described a wheel probe for insertion into the bore of a pipe for the purpose of ultrasonically inspecting the wall of the pipe. The wheel probe comprises a solid annular rim made of a non-deformable material through which sound will pass, and side members which together with the rim define a hollow chamber. One or more ultrasonic transducers are located within the chamber and are positioned adjacent the inside surface of the rim for directing and receiving sound through the rim and a solid resilient tire member mounted on the outside surface of the rim.
In copending U.S. patent application Ser. No. 67,790, filed Aug. 20, 1979, an inspection apparatus is disclosed which includes a vehicle for movement along the bore of a pipe which will cater to bends in the pipe and, in some cases, will accommodate changes in the diameter of the pipe without upsetting, to an unacceptable degree, the alignment, positioning and degree of contact between wheel probes in the bore of the pipe.
It is essential to locate the wheel probes accurately in the bore of the pipe at all times, and in particular the wheel probes must be aligned so that the transducers are positioned at a predetermined angle to the tangent at the wall of the pipe so as to detect the echo or reflections or refractions of sound transmitted by that transducer or another transducer located in the same wheel probe or located in other wheel probes. Hence, it will be appreciated that it is usual to employ a plurality of wheel probes accurately positioned around the bore of the pipe which are arranged to roll along the length of the pipe. This accurate positioning may be achieved using guide wheels as described and claimed in U.S. Pat. No. 4,202,216.
The present invention dispenses with the need for guide wheels, and acknowledges that it is usual to position a plurality of wheel probes around the circumference of the pipe. The present invention also has as an object the provision of inspection devices which are self aligning relative to the pipe.